U.S. patent application Ser. No. 07/532,831, MULTICOLOR IMAGING APPARATUS WITH IMPROVED TRANSFER MEANS, filed in the name of Kevin M. Johnson on Jun. 4, 1990, shows a multicolor image forming apparatus in which a series of electrostatic images are formed on a rotable image member. These images are toned by the application of different color toners to each image to create a series of different color toner images. The images are then transferred in registration to a receiving sheet carried by a transfer drum. The transfer drum and the image member form a nip for the superposition of the images. While images are being transferred, the transfer drum is driven by frictional engagement between the image member and the receing sheet on the transfer drum. To provide precise registration, the image member and transfer drum are kept out of contact between images and the transfer drum is precisely reindexed during this time. As shown in FIGS. 28 and 29 of that application, the separation between the image member and transfer drum is provided by a stop for the transfer drum that maintains a separation between transfer drum and image member less than the thickness of a compressed transfer sheet, against which stop the transfer drum is urged by a force applying means that provides pressure for transfer.
Reindexing is accomplished by a stepper motor which is connected to the shaft of the transfer drum which rotates the transfer drum to a home position between images and then rotates it from the home position in response to a signal indicating the rotational position on the image member of the next toner image to be transferred. At this point the transfer drum is driven by the stepper motor up to a peripheral speed approaching the speed of the image member. When the receiving sheet enters the nip for the next image the rotation of the image member takes over the driving of the transfer drum and the stepper motor can be allowed to move at a non-critical speed at this point. The quality of registration of the toner images is dependent upon the preciseness of this re-contacting between the image member and the receiving sheet.